The present invention relates to a method of, and apparatus for, the measurement the size of an optic zone of a of contact lens or an intraocular lens and/or the power of that optic zone.
The relationship between the central optic zone size of a lens and the patient's pupil size is important in the prescribing of contact lenses and has become more so following the introduction of bifocal contact lenses. Whilst numerous devices are available to measure pupil size accurately, the task of measuring central optic zone diameters precisely can be difficult.
Typically, prior to the present invention, a projection microscope or a hand held measuring magnifier has been used to view the optic zone of a contact lens. However, such devices have two severe limitations. Firstly, the optic zones junctions of bifocal contact lens can only be seen when the difference in refractive power between the two adjacent zones is large. Secondly, these zone junctions can only be seen when they have not been blended i.e. where there is a clear disjunction between the two optic zones. Even a heavy polish without blending can render the optic zone junction invisible using the above devices.
In particular, these limitations have precluded the use of projection microscopes and hand held magnifiers in the observation and measurement of the optic zone and power of bifocal or multifocal hydrophilic lenses. This is so because the difference in power between near and distance optic zone of a bifocal is only a few dioptres. When the lens is immersed in saline solution, as is preferred for hydrophilic lenses, these power differences are decreased and the optic zones are generally undetectable. Moreover, the zone junctions are often blended thereby further reducing visibility of the individual optic zones.